Racial/ethnic differences in breast cancer incidence have been well documented, yet the reasons underlying these differences are only partially understood. Relatively few studies have been conducted in racial/ethnic minority populations and some have produced findings on risk factors that are different from those reported for non-Hispanic White women. Furthermore, most studies in minority populations have assessed risk factors for breast cancer overall, despite growing evidence that breast cancer is a heterogeneous disease, with risk factors that differ for breast cancer subtypes defined by estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor-2 (HER2) status. Most currently known breast cancer risk factors apply to hormone receptor positive (ER+ and/or PR+) or Luminal A (ER+ and/or PR+, HER2) tumors. Few studies have focused on risk factors for the less common subtypes, such as hormone receptor negative (ER- PR-), triple negative (ER-PR-HER2-), or HER2 over-expressing (ER-PR-HER2+) tumors. Reports on risk factors for the less common subtypes were, with a few exceptions, based on small case numbers and included primarily non-Hispanic white women. Given that breast cancer subtypes are not equally distributed across racial/ethnic groups, with a higher incidence of aggressive subtypes in minority populations, it is important that etiologic studies in minority populations assess risk factors in relation to specific breast cancer subtypes. To address this significant gap in knowledge, we will pool existing interview and cancer registry data for 9,000 breast cancer cases and 7,855 controls who participated in five population-based studies. Participants cover a broad age range (18-79 years) and 71% of cases and 66% of controls are racial/ethnic minorities (Hispanics, Asian Americans, African Americans, non-Hispanic whites). In Aim 1, we will evaluate risk factors for breast cancer subtypes and assess heterogeneity by age and menopausal status. Using polytomous logistic regression, we will assess associations of subtypes with a broad set of risk factors, including family history of breast cancer, hormonal factors and modifiable lifestyle factors. In Aim 2, we will assess whether risk factors for the mai subtypes differ across racial/ethnic groups. Leveraging existing data, this study will provide much needed information about risk factors for the less common breast cancer subtypes and will allow direct comparison of subtype-specific risk factors across multiple racial/ethnic groups. Such information will inform the development of preventive strategies that are directly relevant for specific racial/ethnic groups.